1. Field of the Invention
This invention relates generally to a medication infusion device for administering fluid to patients and more particularly to an improved infusion pump with integral flow monitor that is small, inexpensive to manufacture, disposable, and very power efficient.
2. Description of the Related Art
Infusion Devices
Current generation infusion pumps are costly to use. They are difficult to program and require significant resources to properly train medical personnel in their use. The infusion pumps usually require devices that allow the loading and unloading of the cassette and connection to a source of AC power. The pumps require high front-end capital equipment costs and expensive routine maintenance. They typically become obsolete in a few years and must be replaced by newer technology pumps. Pump replacement not only results in high capital equipment costs but also typically requires costly retraining of medical personnel in their use. Investment in these high front-end capital equipment and training costs also forces an unearned “loyalty” to the particular infusion pump provider that further increases the user's costs by stifling competition and restricting the adoption of newer, better, or less expensive infusion pump technologies. Additionally, the disposable cassettes require costly features to precisely interface with the pump and to prevent uncontrolled free flow of fluid to the patient when incorrectly loaded or unloaded. Further, the size and weight of current generation pumps make mobile care difficult and expensive, especially in military applications when they must be transported long distances or in battlefield environments.
As a result of the ongoing need for improved health care, there is a continuous effort to reduce the cost of and to improve the administration of intravenous fluids from infusion devices. As is well known, medication dispensers and infusion devices are used for infusion of predetermined amounts of medication into the body of a patient. Various types of medication dispensers employing different techniques for a variety of applications are known to exist.
Primary types of prior art infusion devices are commonly known as controllers, pumps, disposable elastomeric pumps, and mechanical pumps.
Controllers are infusion devices that control the rate of flow of a gravity infusion. They are limited in use because they are unable to generate positive pressure over and above that provided by gravity. Many infusions require the generation of pressure to overcome pressure losses due to filters or other devices in the fluid path to the patient. Arterial infusions can also require positive pressure to overcome the high blood pressures involved.
Infusion pumps are able to generate positive pressure over and above that provided by gravity and are typically a preferred infusion device. Prior art devices demonstrate a complexity of design in order to sense the presence of tubing, sense the disposable cassette loading operation, control the motor, gear down or reduce the speed of the pumping mechanism, sense upstream and downstream occlusions, and sense the proper operation of the motor. They typically require a complex pumping mechanism with a platen, cams, cam followers, gears or belts, and pressure sensors. The motor drives typically require a costly encoder wheel to sense the position of the motor or cam.
Disposable elastomeric pumps utilize an elastic membrane to form a reservoir to contain and then “squeeze” the medication therefrom. A precision orifice usually controls the rate of infusion. As the elastomeric container empties, the pressure inside can vary significantly which can change the infusion rate. The infusion rate can also vary depending on the viscosity of the infused medication. These devices are typically disposable and utilized for a single infusion.
Mechanical pumps can utilize a spring mechanism in combination with a precision orifice to control the infusion rate. A disposable medication container is loaded into the device. The spring mechanism then squeezes the medication out of the container and through the controlling orifice to the patient. Although mechanical pumps are able to generate positive pressure, they typically cannot detect actual fluid flow nor can they adjust flow rate based on the presence of restrictions in the fluid path. The disposable medication container is used once and discarded after use. Since the infusion rate is dependent on the forces exerted by the spring mechanism, complex mechanisms are required to generate an infusion rate that is accurate from the beginning of the infusion when the reservoir is full to the end of the infusion when the reservoir is empty.
An example of a controller is shown in U.S. Pat. No. 4,626,241 to Campbell et al. The controlling mechanism in this reference can only control the rate of the gravity infusion by repetitively opening and closing a control valve. This device not only has the disadvantages inherent in a controller but also has several other problems in its implementation. The device has limited ability to accurately monitor the volume or rate of the infusion. It uses a drop sensor to count the number of drops infused. It is well known that drop size varies wildly with not only drip chamber canulla size and the rate of infusion, but also with the type of medication being infused.
Another example of a controller mechanism is demonstrated in U.S. Pat. Nos. 4,121,584 and 4,261,356 to Turner et al. This device is further improved in U.S. Pat. No. 4,185,759 to Zissimopoulos, U.S. Pat. No. 4,262,668 to Schmidt, U.S. Pat. No. 4,262,824 to Hrynewycz, and U.S. Pat. No. 4,266,697 to Zissimopoulus. The improved design uses a combination of gravity pressure, a permanent magnet, and an electromagnet to alternately open and close two valves to sequentially fill and empty a fluid chamber. This controller design also operates with gravity flow and has no capability to generate positive fluid pressure as is required in many clinical applications. This design requires a very complex cassette and has no capability to monitor the presence or absence of flow. The presence of an occlusion or empty reservoir cannot be detected by the mechanism. A low head height or low fluid reservoir results in a reduction of the rate of infusion. This type of undetected under-infusion can be hazardous to patient safety.
The implementations of this design in U.S. Pat. No. 4,262,824 to Hrynewycz utilizes the combination of permanent magnets and electromagnets to provide a bistable rocker arm motion to sequentially open and close cassette valves. The permanent magnet(s) are utilized to force one or the other of the two valves to a closed position when power is interrupted, thereby stopping potentially hazardous free flow of fluid to the patient.
The implementation of the design in U.S. Pat. No. 4,266,697 to Zissimopoulos provides a plunger means for the valve members. The design utilizes a very complex combination of magnets, a leaf spring, coil springs, and plungers to implement a bistable valving function that reduces the wear on the valve membrane.
The ability of an infusion pump to generate positive pressure greatly increases its clinical acceptability. Prior art devices, however, demonstrated greatly increased complexity of design. An example of such an infusion pump is in U.S. Pat. No. 6,371,732 to Moubayed et al. The invention includes a variable speed motor with a complex motor speed control, a worm and worm gear, a complex cam and cam follower with roller members and pinch members and pinch fingers and biasing springs. The invention also requires an optical sensor, two pressure sensors with beams and strain gages, a platen sensor, and a tubing sensor. The invention also requires a shut-off valve and an encoder wheel.
An example of a disposable elastomeric pump is shown in U.S. Pat. No. 5,398,851 to Sancoff et al. It can be seen that the shape of the device is bulky and inconvenient for a patient to wear unobtrusively. The device requires an expensive elastomeric membrane to contain the medication and force it through the controlling orifice to the patient. It is disposable and typically filled only once for a single infusion then discarded.
An example of a mechanical pump is shown in U.S. Pat. No. 7,337,922 to Rake et al. It can be seen that the spring mechanism of a preferred embodiment includes two lateral springs and a complex mechanism. Complexity is added to the mechanism to provide a low profile package that is less bulky for the patient to wear. Although large forces are not required to load the infusion reservoir, large forces can be required to force the spring mechanism closed around the reservoir. Additional complexity is added to the mechanism to help reduce the resulting forces and the larger the medication bag, the larger the forces involved. This typically limits the usage of this type of device to fluid reservoirs of a few hundred milliliters or less while many commercially available fluid reservoir bags are one liter in size.
Occlusion Detection Devices
In many cases it is of critical importance to provide an infusion pump that can effectively detect fluid path occlusions either upstream (from the supply reservoir) or downstream (to the patient) in a timely manner. These needs are only partially fulfilled by prior art infusion pumps. Specifically, the occurrence of an occlusion in the pump's medication supply tube or output tube may endanger the patient without warning. If, for example, the supply reservoir is empty, or the supply tube becomes kinked, pinched, or otherwise blocked, the supply of medication to the patient will cease. As the continued supply of some medications is necessary to sustain the patient or remedy the patient's condition, cessation of supply may even be life threatening. Yet, with some infusion devices, such an occlusion would either go unnoticed or require an excessive amount of time to be detected. Some prior art devices such as that described in U.S. Pat. No. 4,398,542 to Cunningham et al. utilize a pressure transducer and membrane to monitor fluid pressure as an indicator of an occlusion.
Still other prior art devices such as that described in U.S. Pat. No. 6,371,732 to Moubayed et al. use strain gages to measure changes in the diameter of tubing as a means of detecting occlusions.
Still other prior art devices as described in U.S. Pat. No. 6,110,153 to Davis et al., utilize a complex optical system to detect changes in the diameter of tubing resulting from upstream occlusions. These devices require costly optical components, expend significant amounts of power to excite the elements, and require precise alignment to operate properly.
Programming Devices
Programming devices for infusion pumps are well known. Devices such as shown in U.S. Design Pat. No. 282,002 to Manno et al. utilize an array of push button switches to select a program value and an electronic display to display the selected value. Devices such as that shown in U.S. Pat. No. 4,037,598 to Georgi utilize switches that can both select the program value and display the selected value on a printed switch assembly. These devices cannot be programmed remotely nor can they be attached or made part of the fluid reservoir.
U.S. Pat. No. 4,943,279 to Samiotes et al. discloses an infusion device that uses an attached magnetic label. The label includes a display of the drug name and concentration with a set of parameter scales that surround the manual controls on the pump when the label is attached. Magnets in the label are sensed by the infusion pump so that it knows the scales and drug information. This device still requires patient specific programming that must be performed at the infusion pump.
The infusion device of U.S. Pat. No. 5,256,157 to Samiotes et al. describes an infusion device that uses replaceable memory modules to configure non-patient specific parameters such as patient controlled analgesia, patient controlled analgesia with a continuous infusion, et cetera. The patient specific programming must then be performed by the user. These replaceable modules do not display either the non-patient specific parameters or the patient specific parameters. Displaying these parameters electronically on the infusion pump requires an increase in cost in the pump and complexity to the operator.